Effect of coil current sweep cycle and temperature change cycle on the screening current-induced magnetic field for Ybco-coated conductor coils

Abstract

YBCO-coated conductors are promising for NMR magnets. Their axial strength is up to 700 MPa and this allows for dramatically smaller high current density magnets. One of the major problems of YBCO-coated conductor coils is screening current induced in the coil during charging. The screening current reduces the central magnetic field by <20% and causes the central magnetic field to drift with time. These are crucial problems for NMR magnets. We investigated measures to counter the field reduction and the drift for YBCOcoated conductor coils, including current cycle and temperature cycle. Experimental results for a small YBCO double pancake coil at 77 K are compared to numerical simulations. The magnitude of the drift increases with coil current. The drift is effectively suppressed by current sweep reversal, but the field reduction is not. A demagnetizing method suppresses both problems although it requires large overshooting in coil current and magnetic field. For a cryocooler cooled YBCO magnet, both problems can be suppressed by current sweep reversal in combination with temperature cycle.